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Environment

Thinning Snows In Rockies Tied To Global Warming

The snowpack in the Rocky Mountains has been gradually thinning over the past century. Using tree ring measurements from subalpine larch trees like these in the Lake Chelan Wilderness in Washington State, researchers were able to put the Rocky Mountain data in long-term historical context.

The snowpack in the Rocky Mountains has been gradually thinning over much of the past century, and a new study attributes much of that to global warming.

This year is a notable exception — unusually heavy snowfall throughout the Rockies this winter has caused a lot of flooding and water-management headaches downstream. But taking the long view, the trend is toward less and less snow.

And snowpack in the Rockies isn't simply of interest to skiers and snowmobilers. "Over 70 million people are dependent on this water," says Greg Pederson from the U.S. Geological Survey in Bozeman, Mont. "This water feeds the Columbia River, the Missouri and the upper Colorado, as well as the Rio Grande."

The Tree Ring Record

Water managers have noticed in recent decades that the snowpack is, on average, getting thinner in the Rockies. Pederson wondered whether that's simply natural variation or if a changing climate means this is a trend that will continue.

"Fifty to 100 years of record is, in many cases, compared to the rest of the Earth's history, a pretty short time interval," he says. "So we were curious how, over the past 500 to 1,000 years, snowpack may have changed."

Obviously people weren't out there taking regular measurements of snowpack in the Rockies hundreds of years ago. So Pederson and his colleagues turned to a natural record of snow depth: tree rings. They used them in two ways. They looked first at trees throughout the West that have fat rings during wet years, signaling high snowpack, and skinny rings during dry years with thin snow. They also looked at trees that are high up in the mountains and are actually stunted by extra snowpack because they don't start growing each spring until the snow that covers them melts away.

Looking back through many centuries of tree rings, Pederson and his colleagues are now able to put the recent decades of reduced snowpack in its historical context.

"The 20th century, across the northern Rockies, looks quite low, on average, compared to the amount of snow that was there over the past millennium," Pederson says. His group's results are published in the current issue of Science magazine.

Part of the reason for the diminished snowpack is the recent global warming trend. He can see the fingerprints of that in the data. Higher temperatures mean that precipitation over the mountains is increasingly falling as rain instead of snow. And even when it falls as snow, it doesn't stick around as long.

"After we get the delivery of snow, we're often times seeing warmer air masses coming in afterwards," he says. "So even if it's dropped as snow, everything's warmer, so it tends to melt faster once the snow is delivered."

'Anything That Impacts The Snowpack Can Also Cause A Drought'

The timing of that melt is critical out West. Wildlife, farmers and anyone else who is thirsty counts on the snowpack to melt gradually through the spring and into the summer in order to provide a steady supply of water. That will apparently dry up sooner and sooner in the coming decades.

"We're going to have to start looking at how we're going to get through potentially warmer and drier summers of the future, without that free bank account of snowpack," Pederson says.

In fact, the situation could lead to drought. That's true not only in the southwestern U.S., where water managers have already started to brace for the worst, but farther north. Drought isn't always caused by a lack of rainfall.

"Here in the western U.S., where we rely really heavily on snowmelt for summer water supply, anything that impacts the snowpack can also cause a drought," says Phil Mote, a climate scientist at Oregon State University. "And what this paper shows is the warming of the 20th century and beyond is already affecting and will profoundly affect the frequency of droughts in the West, simply by whittling away at the snowpack."

He finds the new report persuasive in its link to global warming.

"It's sort of ironic to be talking about this this year, when the Columbia River is at flood stage in Portland," Mote says.

But that underscores the fact that you can't judge the climate by a single year, or even a few decades. That's why the latest research looked back hundreds of years to show that what's happening today really is likely a departure from natural variation.